8358310: ZGC: riscv, ppc ZPlatformAddressOffsetBits may return a too large value

Reviewed-by: eosterlund, mdoerr, fyang

Author: xmas92

src/hotspot/cpu/ppc/gc/z/zAddress_ppc.cpp

@@ -34,9 +34,11 @@
 #include <sys/mman.h>
 #endif // LINUX
 
-// Default value if probing is not implemented for a certain platform: 128TB
-static const size_t DEFAULT_MAX_ADDRESS_BIT = 47;
-// Minimum value returned, if probing fails: 64GB
+// Default value if probing is not implemented for a certain platform
+// Max address bit is restricted by implicit assumptions in the code, for instance
+// the bit layout of ZForwardingEntry or Partial array entry (see ZMarkStackEntry) in mark stack
+static const size_t DEFAULT_MAX_ADDRESS_BIT = 46;
+// Minimum value returned, if probing fail
 static const size_t MINIMUM_MAX_ADDRESS_BIT = 36;
 
 static size_t probe_valid_max_address_bit() {

src/hotspot/cpu/riscv/gc/z/zAddress_riscv.cpp

@@ -36,9 +36,11 @@
 #include <sys/mman.h>
 #endif // LINUX
 
-// Default value if probe is not implemented for a certain platform: 128TB
-static const size_t DEFAULT_MAX_ADDRESS_BIT = 47;
-// Minimum value returned, if probing fails: 64GB
+// Default value if probing is not implemented for a certain platform
+// Max address bit is restricted by implicit assumptions in the code, for instance
+// the bit layout of ZForwardingEntry or Partial array entry (see ZMarkStackEntry) in mark stack
+static const size_t DEFAULT_MAX_ADDRESS_BIT = 46;
+// Minimum value returned, if probing fail
 static const size_t MINIMUM_MAX_ADDRESS_BIT = 36;
 
 static size_t probe_valid_max_address_bit() {